Activated charcoal filter layer for gas masks

ABSTRACT

The activated charcoal filter layer for gas masks contains a highly air-permeable, three-dimensional carrier framework essentially stable in shape, formed of wires, monofilaments or stays, to which a layer of granular, particularly spherical activated charcoal particles with a diameter of 0.1 to 1 mm is affixed. The carrier framework can be composed of wires, monofilaments or stays, the distance between them being at least twice as great as the diameter of the activated charcoal particles. The carrier framework can also be a large-pore reticulated polyurethane foam which has a weight of 20 to 60 g/l and pores with a diameter of 1.5 to 3 mm. The carrier framework can furthermore consist of wire netting or wire mesh arranged in layers, as well as of plastic or wire spirals with a length equal to and up to 10 times their diameter, or of wires or monofilaments with a length of 2 to 10 mm, which are held by twisted metal or plastic wires and project perpendicular from this axis in the shape of a spiral staircase. In spite of a very slight pressure drop, the filter performance is excellent.

All conventional gas mask filters consist of a replaceable filtercartridge which contains at least one activated charcoal layer.Activated charcoal for gas mask filters usually has a specific or"internal" surface of 500 to more than 2000 m² /g, determined accordingto the BET method. It is a particular feature of activated charcoal thatit can permanently and very unspecifically adsorb a large number ofsubstances in its micropores, which can comprise up to 50% of the totalvolume. Toxic gases, e.g. HCN, which are only weakly bound by the normalphysical adsorption, can be bound using metal compounds, e.g. silver,copper or chromium compounds, which are applied, providing superimposedchemical sorption. The activated charcoal filter layer of gas maskfilters is usually formed as a bulk filter, in which the medium to bepurified flows through a fixed bed of the activated charcoal particles.In order to guarantee a sufficient period of functioning of the filter,a sufficient amount, i.e. mass of the adsorber material, must bepresent. At the same time, however, the adsorption kinetics areproportional to the available "external" surface of the particles, sothat small particles are advantageous in this connection. In addition,larger activated charcoal particles can often be fully utilized only intheir outer areas. These are usually already saturated--requiring aninterruption and replacement of the filter cartridge--while the charcoalis only slightly charged on the inside. The use of the smallest possibleparticles in a bulk filter, however, necessarily leads to a highpressure drop. For practical purposes, the particle size is limited in adownward direction by the pressure drop related to it. A furtherdisadvantage of bulk filters is that abrasion phenomena occur as aresult of the activated charcoal particles rubbing against each other,and that the charcoal in powder form increases the flow resistance evenmore.

In general, the opinion is that good filter performance necessarilyrequires high flow-through resistance, because only then can there begood contact between the gas to be purified and the adsorber grains. Inorder to also preclude break-throughs across cavities which form whenparticles settle, the packing must be firmly compressed. In this way,high flow-through resistance of the activated charcoal bulk filters forgas masks is pre-programmed. But this not only results in the effect ofphysiological stress on the gas mask wearer, but also increases thefeeling of constriction.

It is therefore the object of the present invention to create anactivated charcoal filter layer for gas masks, i.e. a gas mask filter,with low flow-through resistance and high adsorption performance.

The solution according to the invention is an activated charcoal filterlayer for gas masks with a highly airpermeable, three-dimensionalcarrier framework essentially stable in shape, formed of wires,monofilaments or stays, to which a layer of granular, particularlyspherical activated charcoal particles with a diameter of 0.1 to 1 mm isaffixed. The distance between the wires, monofilaments or stays is atleast twice as great as the diameter of the activated charcoalparticles. Preferably, it is approximately three to ten times as great.Accordingly, the openings or pores of the carrier framework have adiameter of 1 to 5 mm, preferably 1.5 to 2.5 mm. The diameter of thewires of which the carrier framework can be structured is preferably 0.1to 0.8 mm. If the highly air-permeable, three-dimensional carrierframework, essentially stable in shape, is composed of monofilaments orthreads of plastic, glass or liquefied minerals, the diameter ispreferably 0.2 to 1 mm.

The invention will be further described with reference to theaccompanying drawings, wherein

FIGS. 1 to 5 are views of different carrier frameworks in accordancewith the invention, before and after application thereto of adsorberparticles.

More specifically, the carrier framework can consist of wire netting orwire mesh with a mesh width of 1 to 5 mm, preferably 1.5 to 2 mm,superimposed in layers, between which spacers with approximately thesame length as the diameters indicated are located. Such a carrierframework is shown in FIG. 1, magnified approximately five times,without and with bead charcoal adhering to it.

Another carrier framework which is very useful for the purposes of theinvention consists of plastic or wire spirals with a length equal to andup to approximately ten times their diameter. For example, monofilamentsof thermoplastic synthetics with a diameter of 0.2 to 1 mm can easily beproduced using usual means, e.g. by wrapping them around a mandrel withthe desired inclination. The elasticity is sufficient to strip such aspiral from the mandrel by compressing it, after it has solidified andbeen cut to the desired length. Such carrier structures in the shape ofa spiral generally have a diameter of 0.3 to 10 mm and a length equal toand up to approximately ten times that. Such a carrier structure,without and with charcoal particles affixed to it, is shown in FIG. 2,magnified approximately three times.

Wires or monofilaments with a length of 2 to 10 mm, which are heldtogether by twisted metal wires or plastic monofilaments and thenproject perpendicular from this axis in the shape of a spiral staircase,are also very useful carrier structures for the purposes of theinvention. Such structures are produced in various sizes as test tube orbottle brushes. For the purposes of the invention, they are then cut toa length of several centimeters, e.g. 1 to 10 cm. Filter elements madeof this carrier structure are shown in FIG. 3.

Furthermore, tree-shaped fiber brushes are commercially available, whosefibers, with a length of 1 to 10 mm, radiate out from a center point andare generally attached on a yarn or monofilament, from which they can beremoved after being charged with the activated charcoal particles, e.g.with a suede cutting machine, for the purposes of the invention. Suchcarrier structures are shown in FIG. 4.

A preferred embodiment of the carrier framework is a large-porereticulated PUR foam with a liter weight of 20 to 60 g and pores with adiameter of 1.5 to 4 mm. Such foams are produced in known manner byfirst evacuating a large open-pore foam block located in a closedchamber. Then an explosive gas mixture is allowed to flow in and is thenignited. The cell walls are destroyed by the explosion and melt to formstays. Therefore reticulated foams do not have walls, but rather consistof a grid of stays, which form cages with a diameter of about 1 to 5 mm.These "foams" are elastic, can be easily compressed by hand, even at athickness of several centimeters, and then resume their original shape.When they are completely covered with activated charcoal beads, as shownin FIG. 5, they are relatively rigid and can no longer be compressedwith the same amount of force.

The reticulated foams can already possess the optimal thickness requiredfor production or for use as an activated charcoal filter layer. Inspite of the thickness required, i.e. several centimeters, the open-porestructure permits them to be coated with activated charcoal beads orgrains also on the inside, and they can have the desired shape of theactivated charcoal filter layer of a gas mask directly, or this can bepunched out of a larger plate. It is also possible to subsequently cutup these plates to form elementary filters in the form of strips orchips, with a size of approximately a few cubic centimeters. Thisresults in complete independence from the shape of the objects to befilled, as is also the case for the spirals, fiber brushes or "bottlebrushes" described as carrier structures. The various elementary filterscan be placed in the cavities to be filled, together with heterophilicfibers or threads of hot-melt glue. The entire assembly can besolidified after filling, so that even under great mechanical stress,there is no risk of settling or abrasion, as is the case with bulkfilters.

The possibilities of carrier structures for gas mask filters asdescribed are not to be viewed as limiting. A person skilled in the artcan easily think of other carrier frameworks which can be produced withthe known methods of metal or plastics processing, in a cost-effectivemanner, and which might even occur as waste products. The highlyair-permeable three-dimensional carrier frameworks are supposed to beessentially stable in shape on the basis of the thickness or thestrength of the wires, monofilaments or threads of which they arecomposed, i.e. they should not simply collapse, but can certainly beelastic, so that they can be compressed to a certain extent, but thenresume their original shape. When the granular, especially sphericalactivated charcoal particles are affixed to them and these carrierframeworks are preferably completely covered with the activated charcoalparticles, the rigidity is increased and the filter elements, i.e. theentire activated charcoal filter layers, are then relatively rigid,pressure-resistant structures.

Depending on the material of which the carrier framework is formed, theactivated charcoal particles can be affixed to it directly, or anadhesive mass is required. Plastic materials, particularly fibermaterials, are commercially available, which have the property of firstbecoming sticky on the surface at an elevated temperature, within acertain temperature interval, without melting. This property, whichcould be designated as a built-in hotmelt glue, can be utilized to affixthe activated charcoal particles to them. Such fibers can beheterophilic fibers made of two coaxially arranged components, the outerone of which demonstrates a lower melting point. Unstretched amorphouspolyester fibers which become soft and sticky at approximately 80 to85.C, without melting, and subsequently crystallize at highertemperatures and then have the thermal stability of a normal polyesterfiber, are also suitable. Such fibers with adsorbents affixed to themare described for textile surface filters, for example in DE-AS-32 00959.

Another possibility preferred for the purposes of the invention is toaffix the activated charcoal particles to the carrier framework with anadhesive mass. With this alternative, a person skilled in the art has agreater choice with regard to the material of which the carrierframework is made, as well as with regard to the adhesive mass.

With both possibilities, the diameter of the wires, monofilaments orstays of the surface structure alone or with the adhesive mass isdimensioned in such a way that complete coverage of the carrierstructure with the activated charcoal particles is possible, in order toproduce a carrier framework, i.e. a filter element completely coveredwith the activated charcoal particles, in a preferred embodiment of theinvention.

An activated charcoal filter layer according to the invention can alsobe produced in the following manner: The activated charcoal particlesare preferably affixed continuously on the fibers or monofilaments whichbecome sticky on the surface when heated as described, or on continuouswires or monofilaments which have been coated with an adhesive mass, andthese carriers charged with bead charcoal are cut up into small pieceswith a length of 0.5 to 3 cm. These carriers can, in addition to beingstraight, also be sine-shaped or zigzag-shaped, in order to result in aneven more open structure. When cut up, or later, and possibly incombination with heterophilic fibers or threads of hot-melt glue, anactivated charcoal filter layer with the desired thickness can be formedas an irregular structure of the wires or monofilaments coated with theactivated charcoal, and preferably be compacted or welded in a mold orin the filter cartridge, possibly by being heated.

In order to fix the activated charcoal particles on the carrier, bothinorganic and organic adhesive systems can be used. The latter includepolymers, particularly acrylic acid derivatives, polyurethanes,polystyrenes, polyvinyl acetates as well as hot-melt glues. Those masseswhich consist of polymers which can be cross-linked, which pass througha viscosity minimum before being cross-linked, are preferred. Suchadhesive systems, such as IMPRANIL(®)-High-Solid-PUR reactive productsfrom BAYER(®), are highly viscous at first, i.e. they offer good initialadhesion when the carrier framework is being covered with the activatedcharcoal particles. With an increase in temperature, they demonstrate agreat decrease in viscosity, which results in better wetting of theactivated charcoal particles and therefore especially good adhesionafter hardening, due to cross-linking. When the viscosity minimum isreached, small constrictions form at the contact sites between thecarrier framework and the activated charcoal particles, due to capillaryforces. Because the activated charcoal beads are practically attached atonly one point, almost their entire surface is accessible to the gas tobe cleaned after hardening.

When using metallic or ceramic carriers, adhesive masses of enamel orglazes can be used; in this case, the work has to be carried out in aninert atmosphere, due to the high temperatures required to melt thesecoatings, so that the effectiveness of the activated charcoal particlesis not impaired or destroyed by oxidation.

The activated charcoal particles must be pourable andabrasion-resistant. It is most practical if their diameter is three tofive times smaller than the diameter of the pores or openings of thehighly air-permeable carrier framework. Commercially available activatedcharcoal beads with a diameter of 0.1 to 1 mm are not only the mosteasily pourable form, but also withstand the greatest stress, due totheir symmetry. Granular activated charcoal particles are also suitable,however, as long as they are not too angular or too irregular in theirshape, because it is important that the activated charcoal particles canstill penetrate into structures with a thickness of several centimeterswhen they are affixed on their surface structure.

It is certainly possible to produce activated charcoal filter layersdirectly in the thickness usual for gas masks.

Activated charcoal particles suitable for gas mask filters should havean internal surface of 600 to 2000 m² /g, preferably 1000 to 1600 m² /g,determined according to the BET method. The activated charcoal particlesshould be very pressure-resistant and preferably highly resistant tomoisture. A very abrasion-resistant spherical activated charcoal can beproduced, for example, from coal tar pitch or petroleum distillationresidues. Additional hardening of the surface as well as noteworthymoisture resistance can be achieved with special post-treatment. Theproduction of suitable activated charcoal beads is described, forexample, in EP-B-118 618, DE-B-29 32 571 and DE-A-30 41 115.

In order to increase the abrasion resistance, the activated charcoal canalso be impregnated at its surface in a plastic dispersion or a coal tarpitch solution or bitumen solution, and subjected to slightpost-activation. The sensitivity with regard to steam can besignificantly reduced by adding ammonia gas during post-activation andcooling to 100° C. with exclusion of air.

The activated charcoal particles can contain metal compounds,particularly compounds of the metals silver, copper and chromium. Inaddition, encapsulated enzymes which decompose poisons, as described inEP-B-118 618, can also be present.

With the filters described, excellent separation effects for pollutantsand gases were achieved at extremely low pressure drops. It wassurprisingly shown that it is not necessary for flow to go through theactivated charcoal grains, but rather only past them, in order toachieve high effectiveness with a low pressure drop. The Brownian motionof the gas molecules is sufficient to achieve a high adsorptionvelocity. The pressure drop of an, activated charcoal filter layeraccording to the invention is less than 10 mm, preferably less than 5,and especially less than 2 mm water column at a thickness of 4 cm with acircular cross-section of 100 cm² when an air flow of one liter persecond flows through it. A loosened activated charcoal filter layeraccording to the invention has a greater volume than a bulk filter, withthe same performance, but significantly lower flow-through resistance.The amount of activated charcoal, 100 g, which is usual for a gas masktoday, can be contained in a volume of approximately 350 ml with thecarrier structures according to the invention.

Because of the varied possibilities of structuring the filter materialaccording to the invention, the shape of the gas mask filter can also beadapted to the most varied needs. For example, the filter can certainlybe housed in a hood mask, e.g. around the head or at the neck, and thenserves as additional head or neck protection against impacts. In thiscase, the filtered air should flow past the eyes, in order to preventfogging of the visor window. A plate-shaped filter can be worn on thechest or back and connected with the mask element by way of a flexiblehose. Cylindrical filter elements with a diameter of several centimeterscan also be housed directly in a flexible hose, or be coupled togetherto form a hose, using suitable means. Such replaceable filter elementscan also have various functions. It is most practical if the inletopening of the hoses containing the filter elements or comprised of themis located on the inside of a protective suit.

EXAMPLE 1

Ten wire grids, 50×60 cm, mesh width 2 mm, wire thickness 0.30 mm, wereformed into a package with a height of approximately 4 cm using steeltubes with a thickness of 2 mm as spacers, and dipped into a mixtureconsisting of a "masked" pre-polymer polyurethane (Bayer HS 62), anemulsifier (Bayer HS DW), a cross-linker (Bayer HS C) and water. Theviscosity (approximately 2000 mP.s) was adjusted in such a way that itwas easy to shake off the excess adhesive and the three-dimensionalframework was coated with 440 g adhesive. This was placed in a largebowl and activated charcoal in spherical form (diameter 0.3 -0.6 mm) waspoured over it. Approximately 3 kg adsorber remained in the framework.The adhesive layer was crosslinked by increasing the temperature to 170°C. The activated charcoal beads were fixed very firmly, but only at onepoint, on the carrier framework. In order to measure the airpermeability, an area of approximately 100 cm², sealed laterally, wassubjected to an air flow of 1 1/sec. The pressure drop was less than 2mm water column. 120 mg CCl₄ per liter was also added to the air flow.After passing through the filter material, the CCl₄ concentration wasless than 1 mg/l. Only after adsorption of almost 50 g CCl₄ didbreak-throughs start to be imminent.

EXAMPLE 2

The carrier framework used was a large-pore, reticulated polyurethanefoam, 4 cm thick, with a weight of 80 g/l and a pore diameter of 2.5-3mm. This foam was squeezed with the same adhesive mass as in Example 1(adhesive application, dry 55 g/l), charged with activated charcoalbeads as in Example 1 (approximately 260 g/l) and finally heated to 170°C. for a short time. This resulted in a filter material with a veryslight pressure drop (approximately 1 mm water column at 1 1/sec on 100cm²). The adsorptive properties approximately corresponded to those ofExample 1.

EXAMPLE 3

The same carrier framework as in Example 1 was coated with the slip ofan enamel with a low melting point, and activated charcoal beads weremade to adhere on this. After drying, the filter was subjected tohardening at 650° C., in a nitrogen atmosphere. Such a filter isespecially suitable for high operating temperatures and aggressivesubstances.

It will be understood that the specification and examples areillustrative but not limitative of the present invention and that otherembodiments within the spirit and scope of the invention will suggestthemselves to those skilled in the art.

What is claimed is:
 1. An activated charcoal filter layer for gas masks,comprising a highly air-permeable, three dimensional carrier frameworkessentially stable in shape, formed of wires, monofilaments or stays, towhich there is a affixed a layer of granular activated charcoalparticles with a diameter of about 0.1 to 1 mm and in an amount of 50 to300 g per liter, the distance between the wires, monofilaments or staysbeing at least twice as great as the diameter of the activated charcoalparticles.
 2. A filter layer according to claim 1, wherein the openingsor pores of the carrier framework have a diameter of about 1 to 5 mm. 3.A filter layer according to claim 1, wherein the diameter of the wires,monofilaments or stays of the carrier framework is such that there iscomplete coverage of the carrier framework with the activated charcoalparticles affixed to it.
 4. A filter layer according to claim 1, whereinthe carrier framework is completely covered with the activated charcoalparticles.
 5. A filter layer according to claim 1, wherein the activatedcharcoal particles are affixed to the carrier framework with an adhesivemass.
 6. A filter layer according to claim 5, wherein the adhesive massis a polymer of an acrylic acid derivative, a polyurethane, apolystyrene, a polyvinyl acetate or a hotmelt glue.
 7. A filter layeraccording to claim 5, wherein the adhesive mass comprises a polymerwhich can be cross-linked and which passes through a viscosity minimumbefore being cross-linked.
 8. A filter layer according to claim 5,wherein the adhesive mass comprises an inorganic mixture which can bemelted.
 9. A filter layer according to claim 1, wherein the activatedcharcoal particles have an internal surface of about 600 to 2000 m² /g.10. A filter layer according to claim 1, wherein the activated charcoalparticles are pressure-resistant.
 11. A filter layer according to claim1, wherein the activated charcoal particles are highly resistant tomoisture.
 12. A filter layer according to claim 1, wherein the activatedcharcoal particles are impregnated with a salt of silver, copper orchromium.
 13. A filter layer according to claim 1, wherein the activatedcharcoal particles contain encapsulated enzymes.
 14. A filter layeraccording to claim 1, wherein the carrier framework is plastic.
 15. Afilter layer according to claim 14, wherein the carrier framework is alarge-pore reticulated polyurethane foam.
 16. A filter layer accordingto claim 15, wherein the large-pore reticulated polyurethane foam has aweight of about 20 to 60 g/l and pores with a diameter of about 1.5 to 3mm.
 17. A filter layer according to claim 1, wherein the carrierframework is formed of wire netting or wire mesh superimposed in layers.18. A filter layer according to claim 1, wherein the carrier frameworkis composed of plastic or wire spirals with a length from about 1 to 10times their diameter.
 19. A filter layer according to claim 1, whereinthe carrier structure is formed of wires or monofilaments with a lengthof about 2 to 10 mm, which are held by twisted metal or plastic wiresand project perpendicular from this axis in the shape of a spiralstaircase.
 20. A filter layer according to claim 1, wherein the carrierstructure is formed of fiber brushes which radiate out from a centerpoint, with a length of about 2 to 10 mm.
 21. A filter layer accordingto claim 1, wherein the carrier structures are connected with oneanother by heterophilic fibers or threads of hot-melt glue.
 22. A filterlayer according to claim 1, exhibiting a pressure drop of less than 10mm water column at an air flow of one liter per second for a filterlayer thickness of 4 cm and a circular cross-section of 100 cm².
 23. Ahood mask including a filter layer according to claim
 1. 24. A hood maskaccording to claim 23, wherein the filter layer is formed as a head orneck protection.
 25. A gas mask including a filter layer according toclaim 1 is formed in the shape of a plate, so that it can be worn on thechest or back connected with the mask element by way of a flexible hose.26. A gas mask including a cylindrical filter element according to claim1, with a diameter of several centimeters and arranged in a flexiblehose.
 27. A gas mask comprising a plurality of cylindrical filterelements according to claim 1 arranged to form a hose.